In U.S. Pat. No. 4,966,495 there is further described a floating drilling and production unit in which there is provided an upper part of a tension leg platform. In this design too the top of the tension leg platform is located at surface level. In U.S. Pat. No. 4,702,321 there is described a drilling and production platform with a deep-floating flotation unit from which tension stays extend down to the sea bed and where the floating part is secured by means of a radiating system of anchors.
None of these known structures, however, is intended for use at great depths and under rough weather conditions, nor will it be possible for the structure to be used under such conditions. A tension leg platform of a conventional type which extends over a water depth of, e.g., 800 to 1000 m or more and under rough weather conditions will necessitate the use of a very heavy and very expensive tension leg system. It has further been found that this type of platform has its limitations. One of the difficulties, which is also partially encountered at normal water depths of 3-500 m, is problems in connection with the oscillations to which the tension stays are exposed, including natural frequencies. These will lead to fatigue loads on the stay and are the result, amongst other things, of movements in the water such as wave motion. These phenomena and fatigue loads will be reinforced at depths of the said magnitude of 800 to 1000 m under rough weather conditions and entail a limitation in the use of the tension leg platform under such conditions.
The use of flexible riser pipes will also be restricted at such great depths due to technical conditions which arise from the great hydrostatic pressure, amongst other factors. In any case, flexible risers suitable for this depth will be extremely expensive.